Hezbollah's leader has made a rare public appearance before tens of thousands of protesters in Beirut to rail against the US-made film which denigrates Islam.

Sayyed Hassan Nasrallah told the large crowd in Beirut's southern suburbs that the United States would face more grave consequences if it allowed the release of the full version of the film Innocence of Muslims, which demeans the Prophet Mohammed.

After gunmen killed the US ambassador to Libya and three other Americans last week, Hezbollah criticised the attack as well as the film.

The Beirut protest was probably the largest gathering to oppose the film so far, but it was peaceful and did not go near the US embassy.

Arguing that the world had not yet grasped the depth of hurt felt by Muslims, Nasrallah called on governments to block access to websites showing the film, which was made in California.

"They slandered the purity of his birth, slandered his faith and his morals, slandered his Koran," Nasrallah told tens of thousands of cheering supporters who had marched through southern Beirut's Shiite suburbs.

"The distribution of this entire film must be banned by the Americans," he said.

Nasrallah's emergence from hiding lent more drama to his warnings.

Fearing assassination, the Hezbollah leader has seldom appeared in public since 2006, when the powerful Shiite group's militant wing fought a month-long war with Israel.

He said anger over the film is "not a passing outburst" and called for the international community to agree to criminalise insults against Islam.

He warned of the danger of unleashing further rage if the full-length film emerged.

"America, which uses the pretext of freedom of expression... needs to understand that putting out the whole film will have very grave consequences around the world."

US president Barack Obama's administration has condemned the film as "reprehensible", but said it cannot curb the constitutional right to free speech in America.

The Hezbollah-led protests came after a week of violent demonstrations across the world, in which several US embassies were attacked.